In the past few years, the price of natural gas for use as fuel and chemical feedstock has been steadily increasing. These higher prices have improved the economics of many gas prospects and encouraged the exploration for new reserves of natural gas. Many gas reservoirs, however, have relatively low percentage of hydrocarbons (less than 40% for example) and high percentages of acid gases, principally carbon dioxide, but also hydrogen sulfide, carbonyl sulfide, carbon disulfide, and various mercaptans.
Carbon dioxide acts as a diluent and, in the amount noted above, significantly lowers the heat content of the natural gas. The sulfur-bearing compounds are noxious and may be lethal. In addition, in the presence of water, these components render the gas very corrosive. Clearly, it is desirable to remove acid gases to produce a sweet and concentrated natural gas having a heating value of near 1,000 BTU/SCF either for delivery to a pipeline or conversion to LNG.
The separation of carbon dioxide from methane is difficult and consequently significant work has been applied to the development of methane/carbon dioxide separation methods. These processes can be placed into four general classes: absorption by physical solvents, absorption by chemical solvents, adsorption by solids, and distillation.
Currently, cryogenic distillation is considered one of the most promising methods of separating acid gases, particularly carbon dioxide, from methane. The high relative volatility of methane with respect to carbon dioxide makes such processes theoretically very attractive. However, the methane/carbon dioxide distillative separation has what heretofore has been considered a significant disadvantage in that solid carbon dioxide exists in equilibrium with vapor-liquid mixtures of carbon dioxide and methane at particular conditions of temperature, pressure, and composition. Obviously, the formation of solids in a distillation tower has the potential for plugging the tower and its associated equipment. Increasing the operating pressure of the tower will result in warmer operating temperatures and a consequent increase in the solubility of carbon dioxide, thus narrowing the range of conditions at which solid carbon dioxide forms. However, additional increases in pressure will cause the carbon dioxide-methane mixture to reach and surpass its critical conditions. Upon reaching criticality, the vapor and liquid phases of the mixure are indistinguishable from each other and therefore cannot be separated. A single-tower distillative equilibrium separation operating in the vapor-liquid equilibrium region bounded between carbon dioxide freezing conditions and the carbon dioxide-methane critical pressure line may produce a product methane stream containing 10% or more carbon dioxide. By comparison, specifications for pipeline quality gas typically call for a maximum of 2%-4% carbon dioxide and specifications for an LNG plant typically require less than 100 ppm of carbon dioxide. Clearly, a distillative separation at the above conditions in unacceptable.
Various methods have been devised to avoid the conditions at which carbon dioxide freezes and yet obtain an acceptable separation. Processes which utilize additives to aid in the separation are disclosed in U.S. Pat. No. 4,149,864, issued Apr. 17, 1979, to Eakman et al., U.S. Pat. No. 4,318,723, issued Mar. 9, 1982, to Holmes et al, U.S. Pat. No. 4,370,156, issued Jan. 25, 1983, to Goddin et al, and U.S. Application Ser. No. 532,343, filed Sept. 15, 1983 to the inventors herein.
Eakman et al discloses a process for separating carbon dioxide from methane in a single distillation column. If insufficient hydrogen is present in the column feedstream, hydrogen is added to provide a concentration from about 6 to 34 mole percent, preferably from about 20 to about 30 mole percent. The separation is said to take place without the formation of solid carbon dioxide. The tower pressure is preferably held between 1025 and 1070 psia.
Holmes et al adds alkanes having a molecular weight higher than methane, preferably butane, to the tower feed to increase the solubility of carbon dioxide and decrease its freezing temperature line. The additive n-butane is added in an amount from about 5 moles to 30 moles per 100 moles of feed.
Goddin et al uses a lean oil absorbent, preferably containing butanes and pentanes, to absorb carbon dioxide from a gas mixture, to prevent the formation of solid carbon dioxide in the absorber apparatus, to prevent the formation of an azeotrope of carbon dioxide with ethane, and to enhance the separation of any hydrogen sulfide from carbon dioxide.
Valencia et al teaches the addition of a light gas, such as helium, to a cryogenic distillation tower to raise the critical pressure of the carbon dioxide-methane mixture therein and allow distillation at a pressure above the critical pressure of methane while avoiding the formation of solid carbon dioxide.
As illustrated by the previous references, the distillative separation of methane and carbon dioxide has heretofore been hindered by the existence of solid carbon dioxide in equilibrium with vapor-liquid mixtures of carbon dioxide and methane at particular conditions of temperature, pressure and composition. Formation of solid carbon dioxide plugs currently employed distillation equipment. Therefore, it has been the teaching that the formation of solid carbon dioxide should be avoided in a fractional distillation process. As discussed above, solid carbon dioxide formation has been avoided by stopping the fractional distillation process before product streams of desired purity were produced requiring further separation by other means. Solid carbon dioxide formation has also been avoided by adding a third component to the fluids being separated by distillation requiring subsequent removal of such third component. Therefore, the need exists for a distillative methane-carbon dioxide separation process which can achieve desired product purity without avoiding solid carbon dioxide formation or adding a third component to the separation process.